1. Field of the Invention
The present invention pertains to cellular telecommunications, and particularly to determining broadcast power to be used by a mobile user equipment unit when requesting access to a cellular telecommunications network.
2. Related Art and Other Considerations
Cellular telecommunications systems employ a wireless link (e.g., air interface) between the (mobile) user equipment and a base station (BS). The base station has transmitters and receivers for radio connections with numerous user equipment units. One or more base stations are connected (e.g., by landlines or microwave) and managed by a radio network controller (also known in some networks as a base station controller [BSC]). The radio network controller is, in turn, connected through control nodes to a core communications network. Control nodes can take various forms, depending on the types of services or networks to which the control nodes are connected. For connection to connection-oriented, switched circuit networks such as PSTN and/or ISDN, the control node can be a mobile switching center (MSC). For connecting to packet switching data services such as the Internet (for example), the control node can be a gateway data support node through which connection is made to the wired data networks, and perhaps one or more serving nodes.
When a mobile user equipment unit desires to initiate a connection with a base station, the user equipment unit typically sends a connection request message to the base station on a Random Access Channel (RACH). The RACH is a common channel that is available for all user equipment units within a geographical area or cell covered by the base station. The connection request message carried on the RACH generally includes various types of other information, such as (for example) signal strength of the received broadcast channel (so that the base station can calculate the transmission power to use) and some random number for (with probability) making the connection unique in the cell during the initial communication on the common channels (RACH and FACH). It is anticipated that the RACH may also be used for packet data in new telecommunications systems, such as wideband CDMA (WCDMA).
When a mobile user equipment unit sends its connection request message on the RACH, the user equipment unit must first calculate the transmit power that its transmitter will use for sending the connection request message. Preferably, in calculating the transmit power, the user equipment unit will use a transmission power which (hopefully) will be the lowest possible transmission power possible, thereby reducing interference for other users. But if the base station fails to receive the connection request message, the user equipment unit must repeat the transmission of its connection request until the base station successfully receives the connection request message, else the attempt to send the connection request is terminated as being unsuccessful.
According to a current technique, if the user equipment unit calculates too low a transmission power (thereby making it impossible for the base station to receive the connection request message), the user equipment unit increases its transmit output power for a successive connection requests (e.g., for each new connection request attempt). However, this current technique presents numerous problems.
A first set of problems can occur when the initially calculated transmit power for the connection request is too low. In such case, the power calculation for transmission of the next connection request, although increased, will probably also be too low. Moreover, the first connection request, sent at a transmit power too low to be received by the base station, can exacerbate the situation by increasing the interference for other user equipment units (perhaps causing them to increase their respective transmit powers) and consuming battery power in the requesting user equipment unit. Further, the delay in sending the next connection request message will likely be increased.
A second set of problems can occur when the initially calculated RACH transmit power is higher than necessary. In such case, the connection request message, sent at a greater-than-necessary power, increases interference for other users. In addition, the transmission of the connection request message consumes more power than necessary in the requesting user equipment unit.
A third set of problems involves aging of the user equipment unit. As a user equipment unit ages, under otherwise identical conditions the user equipment unit can require a different transmit power than when it was new due, e.g., to changing characteristics of the transmitter and/or receiver components. As a result, the user equipment unit will likely use too high or too low a transmit power for its first connection request.
Prior art RACH power determination techniques are discussed in the patent literature. See, for example, U.S. Pat. No. 5,806,003 to Jolma et al.; U.S. Pat. No. 5,487,180 to Ohtake; and U.S. Pat. No. 5,333,175 to Ariyavisitakul et al.
What is needed, therefore, and an object of the present invention, is a technique which is adaptive for changing characteristics of the mobile user equipment unit for determining transmit power when making a connection request.
A transmit power value (PRACH) for a connection request message (RACH) from a mobile user equipment unit to a telecommunications network is determined using a target ratio of received energy to interference value [(Eb/I0)RACHxe2x80x94target] and a modification factor [(Eb/I0)MEANxe2x80x94DELTA]. The modification factor is a sliding mean differential between the target ratio and an actual ratio [(Eb/I0)ACTUAL] of received energy to interference as determined by the telecommunications network. The modification factor allows for the calculation of the transmit power for the connection request message to be adaptive with respect to a changing characteristic of the mobile user equipment unit (e.g., aging of the mobile user equipment unit).
The modification factor is obtained by taking a mean of a set of differentials [(Eb/I0)DELTA]. Each differential of the set is a difference between the target ratio and the actual ratio. Plural differentials of the set are compiled with respect to a corresponding plural number of previous connection request messages. Preferably the set of differentials are stored in a FIFO, so that a sliding mean of the plural (n) differential values maintained in the FIFO can be determined and used as the modification factor [(Eb/I0)MEANxe2x80x94DELTA].
Determining the transmit power value for the connection request message comprises evaluating the expression PRACH=CRACH+(Eb/I0)ADAPTED TARGET+IUL+Pperchxe2x80x94sentxe2x88x92Pperchxe2x80x94meas, wherein PRACH is transmit power for the connection request message calculated by the on-board unit; CRACH is a constant depending on rate, channel coding, and carrier bandwidth; IUL is uplink interference determined at the telecommunications network; Pperchxe2x80x94sent is downlink output power used for a broadcast channel; Pperchxe2x80x94meas is received broadcast channel power measured at the user equipment unit; (Eb/I0)ADAPTED TARGET is an adaptive target Eb/I0 for the connection request message.
The value (Eb/I0)ADAPTED TARGET is determined using the target energy to power ratio (Eb/I0)RACHxe2x80x94target as specified by the telecommunications network and the modification factor. The uplink interference (IUL), downlink output power for the broadcast channel (Pperchxe2x80x94sent), and target energy to power ratio [(Eb/I0)RACHxe2x80x94target] are transmitted to the mobile user equipment unit in a broadcast channel message. After the mobile user equipment unit successfully completes the connection request message, the telecommunications network sends a access grant message which includes an actual ratio of received energy to interference [(Eb/I0)ACTUAL] determined by the network for use as a latest differential affecting the modification factor for the next connection request message from the mobile user equipment unit.